well, i got 22.5 minutes, because to get 5 peices from a plank of wood, you do 4 cuts. it takes him 10 mins.
10 divided by 4 = 2.5
so it takes 2.5 minutes per cut.
then to get 10 peices, you need to do 9 cuts.
2.5 multiplied by 9 = 22.5
thats how i got it, how did you get 20?